The present invention relates to a method of preventing the oxidation of austenitic stainless steel material in high temperature steam and more particularly it relates to a method of preventing the oxidation in high temperature steam of the inner surface of a superheater tube or reheater tube for a boiler made from such material.
With recently constructed large boilers for thermal power generation, austenitic stainless steel is used as the material for the tubes of high-temperature, high-pressure steam generating superheaters or reheaters. When the material is oxidized in high temperature steam, the thickness of the scale formed on the material is far greater than that of the scale which will be formed on the material when it is heated and oxidized at the same temperature in the atmosphere and this scale grows with the time. For example, the thickness of scale formed on the inner surface of the final stage tube (SUS 321 HTB) in a superheater operated at the outlet temperature of 569.degree. C and gauge pressure of 174 Kg/cm.sup.2 is between 50 and 70 .mu. after one year's service and on the order of 100 .mu. after three year's service. This scale is usually composed of two layers of inner and outer layers with the outer layer consisting of iron oxide containing practically no such alloying elements as Cr, Ni and Mn, whereas the inner layer contains large amounts of these alloying elements in concentrated form. Because of these differences in chemical composition and physical properties between the outer and inner layers, the outer layer of the scale tends to easily break away during the stopping periods of the boiler operation as well as during the starting periods of the boiler operation. Thus, if there is a large amount of such separated scale, the separated scale accummulates in the tubes with the resulting clogging of the tubes at the bends, etc., or the separated scale is carried away from the tubes by a high velocity stream of steam causing abnormal wear of the various valves and turbine blades.
Particularly, in the case of a suspension type superheater, the scale tends to accumulate in the lower bend portions with the result that the accumulated scale obstructs the flow of steam and the temperature of the tube walls increases abnormally thus involving the danger of causing a bursting boiler failure. In an attempt to prevent the occurrence of such failures, presently a difficult operation has been employed in which the clogging of the tubes by the scale is detected by means of a .gamma. ray inspection during the periodic inspection so that any tube portions with a large amount of accumulated scale is cut off thus removing the scale and clearing the tubes and then the cut off tube portions are rejoined by welding. With such a troublesome operation, it is still impossible to completely prevent the occurrence of bursting boiler failure. Further, such failure may also be caused by abnormal wear of the valves, etc., due to the scale conveyed along with the flow of steam and therefore the accumulation of separated scale due to oxidation of the tubes involves the danger of causing a serious damage. However, no practical method of effectively preventing the formation of scale has been put in use as yet.